1. Field of the Invention
This invention relates to steam turbines, and in particular, to an improved method of erosion control for rotating blades in the low pressure stage of a steam turbine.
2. Description of the Prior Art
In a low pressure stage of a steam turbine, a significant portion of the steam flowing through the turbine is in a liquid state. This fog of minute water droplets, suspended and carried by the steam flow, presents no hazard to the rotating blades when carried by the steam flow. However, these water droplets have been observed to accumulate on the outer reaches of stationary blading in the low pressure stages, and by so accumulating, form comparatively larger drops of water which are then torn from the stationary blades and impinge upon the leading edges of the rotating blades. Impingement of these relatively heavier water drops upon the leading edges of the rotating blades leads to what is known in the art as erosion.
Most of the erosion that has been observed occurs along the backside of the leading edge of the rotating blades. The erosion on the rotating blades becomes more severe as the blade tip is approached. At the tip, the erosion has been observed to be of such a magnitude as to cut a triangular notch into the rotating blade which threatens to sever the tip portion from the air foil portion of the blade.
In general, it is known that individual water droplets carried by the steam flow are unlikely to find a mechanism for agglomerating to a drop size able to cause erosion damage. It is obvious therefore that water droplets must collect in a more massive form on the stationary blade surfaces and then be pulled therefrom to produce drops of sufficient size to cause erosion of the rotating blades. The small droplets which are carried by the steam flow collect on the stationary blades to provide the source for the relatively larger drops.
There is generally considered two mechanisms of collection of the small water droplets on the stationary blades. The first mechanism is known to those skilled in the art as the direct impaction mechanism, while the second is known as the diffusion mechanism.
The direct impaction mechanism utilizes the momentum and inertia of relatively large water droplets to directly impact upon the surfaces of the stationary blades. The impaction mechanism is effective only for droplets greater than 1 micron in size. A micron is one-millionth of a meter. Once deposited, drops torn from the stationary blades by the steam flow cause severe damage as they impinge on the rotating blades near the rotating blade tips. However, the erosion damage throughout the radial length of the rotating blades is due to the deposition of fine water droplets carried in the steam flow on the stationary blades.
Since it has been shown that collection by the impaction mechanism does not result in the deposition of the fine water droplets carried by the flow on the stationary blades, the second well-known mechanism, that is, the diffusion mechanism, provides the source for the deposit of fine water droplets on the stationary blades. There are two types of diffusion processes well known in the art, the Brownian diffusion process and the eddy impaction process.
The Brownian diffusion process is a modification of the known mass transfer phenomena of eddy diffusion and molecular diffusion. When analyzed in terms of the problem under consideration, it has been found that the molecular diffusion phenomenon controls the deposition of the small water droplets close to the surface of the stationary blades, while the eddy diffusion phenomenon controls a distance away from the stationary blade surface.
The Brownian diffusion process is especially effective for water droplet sizes smaller than one-tenth of one micron.
When the droplet size becomes too large for Brownian diffusion yet too small to generate sufficient momentum to be controlled by the impaction mechanism, the second type of the diffusion mechanism, the eddy impaction process, becomes controlling. The process of eddy impaction occurs where the comparatively small water droplets, sized between one-tenth to one micron, have approached closely to the surface of the stationary blades in a manner described by the eddy diffusion phenomenon mentioned above. The momentum developed by these small water droplets is dissipated by viscous resistance as they travel to the stationary blade surface. The droplet velocity is essentially zero just before collection on the stationary blade surface. In this manner, the water droplets are deposited upon the stationary blade surface.
Collating, for droplet sizes greater than one micron, the direct implection mechanism best explains the collection process. However, the droplets so collected are dragged downstream on the stationary blade and drops torn therefrom primarily cause damage to the rotating blade tips. Damage to the radial length of the rotating blade occurs from water droplets collected on the stationary blading by either the Brownian diffusion process, for droplets less than one-tenth of a micron in size, or by the eddy impaction process, for droplets greater than one-tenth yet less than one micron in size.
Whatever mechanism utilized, the result is the agglomeration of water droplets from the wet steam on the stationary turbine blades. The droplets collected from the wet steam form larger drops of water which are swept from the stationary blades by the generally axial flow of steam.
Since the drops which have been torn from the stationary blades are relatively large and move at a velocity slower than the velocity of the rotating blade tip, the water drops impact on the backside of the rotating blades and result in erosion of the rotating blades, thus causing severe damage to the rotating blade, and a loss of energy and lowering of the efficiency of the low pressure stage.
It has been a practice in the prior art to coat the last several rows of rotating blades with a hard material, such as stellite. However, providing an erosion resistant material on the rotating blades is an expensive process and is not always adequate to overcome the erosive effects due to the impingement of water drops on the blades.
The prior art has also protected the rotating blades of low pressure stage by providing a suction slot adjacent the trailing edge of the stationary nozzle blades and connecting the slots directly to the low pressure condenser to draw the water deposited on the stationary blades directly to the condenser. It has also been found that an increase in the axial spacing between the rotating blades and the stationary blades will increase the velocity of the water drops and allow them to impact upon the leading edges of the rotating blades. However, this method of protecting the last row of rotating blades increases the turbine length, weight, and incidentially thereto, the cost.